Study On Reliability Modeling And Analysis Of CNC Machine Tools Based On Maintenance Degree

As the basic unit of manufacturing system, CNC machine tools is a technology- integrated manufacturing equipment. With the function diversifies, especially for the high-speed &high-precision, the reliability problem which describes the sustainability of the function arised. Nowadays, the function gap of the domestic and the overseas CNC machine tools is decreased gradually. However, the reliability gap is obvious. The reliability problem has become the bottle-neck for the domestic CNC machine tools industry and attracted the attentions of the researchers and engineers. Reliability modeling is a significant work for the CNC machine tools reliability research, whether the model is accurate or not is the assurance and premise for the reliability design and growth. So far, limited to the convenience of the data accumulation and reliability modeling, the Weibull and Exponential distribution are common used statistical models and the as-good-as-new is the general assumption. However, the repair effect is between as-good-as-new and as-bad-as-old to the complex system such as CNC machine tools, So, the repair effect must be considered for the reliability modeling .The paper focuses on the repair degree to the CNC machine tools and its'features, and the reliability modeling method was proposed based on NHPP and incomplete repair circumstances. Then, the failure modes and the availability under different repair degree is was studied and the specific work include:(1) This paper first describes the domestic and international research progress, and presents a new modeling method of CNC machine tools based on various maintenance degrees considering the practical needs of the subject about reliability design of CNC machine tools and key components.(2) This work sets up the mathematical model of the whole CNC machine tools and solves it according to the assumption of the minimum maintenance, and put forward the mathematical model of the non-homogeneous Poisson process of CNC machine based on the theoretical analysis on repairable system. Aiming at failure data for the field test having traits such as multi-sample and random censoring, total failure time method is used to pre-treat, based on these, depending on the different fault tread, reliability model based on weibull process and twofold weibull process is put forward of which the model parameters are determined by maximum likelihood estimation. The paper establishes reliability modeling of the fault data for CNC lathes and machining centers in a machine tool plant, the result of which has better goodness of fit and is more suitable to engineering condition of the CNC machine tools as repairable system through the relevant inspection(3) The reliability assessment of the key components is one of the key links of assessing the reliability of CNC machine tools. A new method of modeling the reliability of the key components with incomplete maintenance is proposed considering that the maintenance degrees'impacts on the key components is between the two states"as-good-as-new"and"minimum maintenance". This model is established based on the Kijima I incomplete maintenance model, of which the hazard rate curve had the"bathtub"form. Meanwhile, the model's parameters are calculated using the method integrating maximum likelihood method and genetic algorithm. The results shows that: this model can not only get the critical values of early-failures period and random-failures period, but also the maintenance degrees within the random-failures period and wear out-failures period, reflecting the real states of each sub-system's changes in reliability.(4) Based on the reliability models of the CNC machine tools and the key components, the failure modes of the CNC machine tools are researched, and the defects of the FMECA not considering maintenance degrees are analyzed, then the improved FMECA is proposed considering maintenance degrees. This method integrated the results of qualitative analysis and quantitative analysis, established the algorithm based on weighted Euclidean Distance and Risk Space Diagram, thus avoiding the amplification effect caused by the multiplication of the components of PRN in the traditional methods. Meanwhile, the maintenance degree is an important factor influencing the failure modes, thus it is helpful to introduce it into the quantitative analysis of criticality in order to fully research the failure modes. Through the failure mode analysis of CNC lathe, the weak link of the machine can be found out, thus providing critical references for reliability-design and reliability growth.(5) In the end,the availability mathematical model concerned the CNC machine maintenance is researched. According to the data of corrective maintenance, author puts forward the availability analysis method based on Monte Carlo. And the simulations of the whole machine and subsystem are made under the assumption of the minimum maintenance and imperfect maintenance. The results show that the availability model describes the instantaneous availability curve from which author gets the law of CNC machine availability. And it consummates the availability analysis method of CNC machine.The research rationalizes the reliability model of the CNC machine and subsystem, enriches the availability analysis method of CNC machine and improves the accuracy of the reliability evaluation. And it puts forward a new idea and method of the reliability analysis. The innovations of this thesis are mainly the following aspects:(1) The minimum maintenance modeling idea is used in the reliability modeling of CNC machine based on Non-Homogeneous Poisson Process. The mathematical model is made under the assumption of repair as-bad-as-old. The method has a good goodness-of-fit and provides a new solution for the whole machine modeling of CNC machine.(2) Establish the incomplete repair model of the key functional components. It gets the critical value and maintenance degree of the early failure period and occasional failure period. And the results can reflect the life stage characteristic of each key functional component.(3) In this paper, the maintenance degree is regarded as one of influence factors. Furthermore, the RPN obtained by qualitative analysis and criticality obtained by quantitative analysis are organically combined, and a mathematical method for comprehensive risk assessment is proposed, which could make the fault analysis results for CNC machine tools repairable systems accords with actual situation.(4) In view of the availability problem for CNC machine tools, the availability function is discussed, and a Monte-Carlo based availability mathematical model in different maintenance degree was built. In this way, we could get the availability curve in different maintenance degree, which make a new extension for available analysis methods of CNC machine tools.